On-board aircraft electrochemical system

ABSTRACT

An onboard electrochemical system of an electrochemical cell including a cathode and an anode separated by an electrolyte separator is selectively operated in either of two modes. In a first mode of operation, water or air is directed to the anode, electric power is provided to the anode and cathode to provide a voltage difference between the anode and the cathode, and nitrogen-enriched air is directed from the cathode to an aircraft fuel tank or aircraft fire suppression system. In a second mode of operation, fuel is directed to the anode, electric power is directed from the anode and cathode to one or more aircraft electric power-consuming systems or components, and nitrogen-enriched air is directed from the cathode to a fuel tank or fire suppression system.

BACKGROUND

This disclosure relates to aircraft systems, and in particular to anon-board aircraft electrochemical system.

It is recognized that fuel vapors within fuel tanks become combustiblein the presence of oxygen. An inerting system decreases the probabilityof combustion of flammable materials stored in a fuel tank bymaintaining a chemically non-reactive or inert gas, such asnitrogen-enriched air, in the fuel tank vapor space also known asullage. Three elements are required to initiate and sustain combustion:an ignition source (e.g., heat), fuel, and oxygen. Combustion may beprevented by reducing any one of these three elements. If the presenceof an ignition source cannot be prevented within a fuel tank, then thetank may be made inert by: 1) reducing the oxygen concentration, 2)reducing the fuel concentration of the ullage to below the lowerexplosive limit (LEL), or 3) increasing the fuel concentration to abovethe upper explosive limit (UEL). Many systems reduce the risk ofcombustion by reducing the oxygen concentration by introducing an inertgas such as nitrogen-enriched air (NEA) to the ullage, therebydisplacing oxygen with a mixture of nitrogen and oxygen at targetthresholds for avoiding explosion or combustion.

It is known in the art to equip aircraft with onboard inert gasgenerating systems, which supply nitrogen-enriched air to the vaporspace (i.e., ullage) within the fuel tank. The nitrogen-enriched air hasa substantially reduced oxygen content that reduces or eliminatescombustible conditions within the fuel tank. Onboard inert gasgenerating systems typically use membrane-based gas separators. Suchseparators contain a membrane that is permeable to oxygen molecules, butrelatively impermeable to nitrogen molecules. A pressure differentialacross the membrane causes oxygen molecules from air on one side of themembrane to pass through the membrane, which forms oxygen-enriched air(OEA) on the low-pressure side of the membrane and NEA on thehigh-pressure side of the membrane. The requirement for a pressuredifferential necessitates a source of compressed or pressurized air.Bleed air from an aircraft engine or from an onboard auxiliary powerunit can provide a source of compressed air; however, this can reduceavailable engine power and also must compete with other onboard demandsfor compressed air, such as the onboard air environmental conditioningsystem. Moreover, certain flight conditions such as during aircraftdescent can lead to an increased demand for NEA at precisely the timewhen engines could be throttled back for fuel savings so that thatmaintaining sufficient compressed air pressure for meeting the pneumaticdemands may come at a significant fuel burn cost. Additionally, there isa trend to reduce or eliminate bleed-air systems in aircraft; forexample Boeing's 787 has a no-bleed systems architecture, which utilizeselectrical systems to replace most of the pneumatic systems in order toimprove fuel efficiency, as well as reduce weight and lifecycle costs. Aseparate compressor or compressors can be used to provide pressurizedair to the membrane gas separator, but this undesirably increasesaircraft payload, and also represents another onboard device with movingparts that is subject to maintenance issues or device failure.Additionally, the membranes in such gas separators are subject tofouling over time.

The concern with combustion as a significant risk management issue foraircraft is not limited to the fuel tanks, and commercial and militaryaircraft are often equipped with fire suppression technology such ashalocarbon systems that disperse a halocarbon such as Halon 1301 as aclean fire suppressant. Halocarbons interrupt the chain reactions thatpropagate the combustion process. Unfortunately, although halocarbonsare deleterious to the ozone layer and are furthermore greenhouse gases,it has been difficult to discontinue their use because of a dearth ofviable alternatives. Typically multiple tanks of Halon are on boardaircraft for fire suppression with respect to both the initial inrush(knockdown) as well as for the replacement of Halon and air lost toleakage at a low rate of discharge (LRD).

BRIEF DESCRIPTION

According to some embodiments of this disclosure, an onboard aircraftelectrochemical system comprises an electrochemical cell comprising acathode and an anode separated by an electrolyte separator. A cathodefluid flow path is in fluid communication with the cathode, anddischarges nitrogen-enriched air. A nitrogen-enriched air flow pathreceives nitrogen-enriched air from the cathode fluid flow path anddelivers it to a fuel tank, a fire suppression system, or to both a fueltank and a fire suppression system. A first anode flow path isconfigured to controllably direct water or air to the anode. A secondanode flow path is configured to controllably direct fuel to the anode.An electrical connection is in controllable communication between anelectric power source and the cathode and anode, and an electricalconnection is in controllable communication between an electric powersink and the cathode and anode. A controller is operatively connected tovarious components of the system and is configured to alternativelyoperate the electrochemical cell in either of two modes. In a firstoperation mode, water or air is directed to the anode from the firstanode flow path, electric power is directed from the power source to theanode and cathode to provide a voltage difference between the anode andthe cathode, and nitrogen-enriched air is directed from the cathode tothe fuel tank or the fire suppression system. In a second mode, fuel isdirected to the anode from the second anode flow path, electric power isdirected from the anode and cathode to the power sink, andnitrogen-enriched air is directed from the cathode to the fuel tank,fire suppression system, or fuel tank and fire suppression system.

In some embodiments of the disclosure, a method of operating an on-boardaircraft electrochemical system comprises selectively operating anelectrochemical cell comprising a cathode and an anode separated by anelectrolyte separator in either of two modes. In a first mode ofoperation, water or air is directed to the anode, electric power isprovided to the anode and cathode to provide a voltage differencebetween the anode and the cathode, and nitrogen-enriched air is directedfrom the cathode to a fuel tank or a fire suppression system. In asecond mode of operation, fuel is directed to the anode, electric poweris directed from the anode and cathode to one or more on-board electricpower-consuming systems or components, and nitrogen-enriched air isdirected from the cathode to the fuel tank, the fire suppression system,or the fuel tank and fire suppression system.

BRIEF DESCRIPTION OF THE DRAWINGS

Subject matter of this disclosure is particularly pointed out anddistinctly claimed in the claims at the conclusion of the specification.The foregoing and other features, and advantages of the presentdisclosure are apparent from the following detailed description taken inconjunction with the accompanying drawings in which:

FIG. 1 is a schematic depiction of an example embodiment of anelectrochemical system;

FIG. 2 is a schematic depiction of a first operating mode as describedherein;

FIG. 3 is a schematic depiction of a second operating mode as describedherein;

FIG. 4 is a schematic depiction of a solid oxide electrochemical cellstack with cooling gas flow through bipolar separator passages;

FIG. 5 is a schematic depiction of a solid oxide electrochemical stackutilizing endothermic steam reforming for cooling; and

FIG. 6 is a schematic depiction of integration of a solid oxideelectrochemical stack integrated with a turbocompressor.

DETAILED DESCRIPTION

Referring now to the Figures, in which the same numbering may be used inmore than one Figure to represent the same feature without the necessityof explicit repetition of the description for each Figure, FIG. 1schematically depicts an electrochemical cell 10. The electrochemicalcell 10 comprises an electrolyte 12 having a cathode 14 disposed on oneside and an anode 16 disposed on the other side. Cathode 14 and anode 16are positioned adjacent to, and preferably in contact with theelectrolyte 12 and can be solid metal layers deposited (e.g., by vapordeposition) onto the electrolyte 12, or can have structures comprisingdiscrete catalytic particles adsorbed onto a porous substrate that isattached to the electrolyte 12. Alternatively, the catalyst particlescan be deposited on high surface area powder materials (e.g., graphiteor porous carbons or metal-oxide particles) and then these supportedcatalysts may be deposited directly onto the electrolyte 12 or onto aporous substrate that is attached to the electrolyte 12. Adhesion of thecatalytic particles onto a substrate may be by any method including, butnot limited to, spraying, dipping, painting, imbibing, vapor depositing,combinations of the foregoing methods, and the like. Alternately, thecatalytic particles may be deposited directly onto opposing sides of theelectrolyte 12. In either case, the cathode and anode layers 14 and 16may also include a binder material, such as a polymer, especially onethat also acts as an ionic conductor. In this case, the cathode andanode layers 14 and 16 may be cast from an “ink,” which is a suspensionof supported (or unsupported) catalyst, ionomer, and a solvent that istypically an aqueous solution (e.g., a mixture of alcohol(s) and water)using processes that are like those used to make catalyst layers used inconjunction with electrolytes in fuel cells. Cathode 14 and anode 16 canbe fabricated from catalytic materials suitable for performing theneeded electrochemical reaction (e.g., the oxygen-reduction reaction onthe cathode and the electrolysis of water on the anode). Exemplarycatalytic materials include, but are not limited to, platinum,palladium, rhodium, carbon, gold, tantalum, tungsten, ruthenium,iridium, osmium, alloys thereof, and the like, as well as combinationsof the foregoing materials.

The cathode 14 and anode 16 can be controllably electrically connectedby electrical circuit 18 to an electric power source 20 (e.g., DC powerrectified from AC power produced by a generator powered by a gas turbineengine used for propulsion or by an auxiliary power unit) or to anelectric power sink 22 (e.g., one or more electricity-consuming systemsor components onboard the aircraft), or power bus(es) for such on-boardelectricity-consuming systems or components. In some embodiments, theelectrical circuit 18 can connect to an electrical power sink 22 in theform of an on-board DC power bus (e.g., a 28 volt power bus commonlyused on commercial aircraft). In some embodiments, the electricalcircuit 18 can be connected to an electrical power sink 22 in the formof an on-board AC power bus (or a portion of an AC bus) through a powerconditioner that includes an inverter (not shown) that converts DC powerproduced by the electrochemical cell to AC power that may be required byaircraft systems or components. Control of this connection can beimplemented through electrical switches (not shown).

A cathode fluid flow path 24 directs air from an external source (e.g.,fan, compressor, ram air flow) into contact with the cathode 14. Oxygenis electrochemically depleted from air along the cathode fluid flow path24, and is discharged as nitrogen-enriched air (NEA) at cathode exhaust26 for delivery to an on-board fuel tank (not shown), or to an aircraftfire suppression system associated with an enclosed space (not shown),or controllably to either or both of an aircraft fuel tank or anon-board fire suppression system. An anode fluid flow path 28 isconfigured to controllably receive water along a first anode flow path30 if the electrochemical cell is configured for proton transfer acrossthe electrolyte 12 (e.g., a proton exchange membrane (PEM) electrolyteor phosphoric acid electrolyte), or to receive cooling air along theanode flow path 30 if the electrochemical cell is configured for oxygenanion transfer across the electrolyte 12 (e.g., a solid oxideelectrolyte). The anode fluid flow path 28 is also configured tocontrollably receive fuel (e.g., hydrogen for a proton-transfer cell,hydrogen or hydrocarbon reformate for a solid oxide cell) along a secondflow path 32. Anode exhaust 34 can, depending on the type of fuel celland the anode exhaust content, be exhausted or subjected to furtherprocessing. Control of fluid flow along these flow paths can be effectedthrough conduits and valves (not shown).

A controller 36 is in operative communications with valves, pumps,compressors, or other fluid flow components and with switches and otherelectrical system components to selectively operate the electrochemicalcell in either a first mode or a second mode. These control connectionscan be through wired electrical signal connections (not shown) orthrough wireless connections. A first operational mode, which can alsobe referred to as an electrolyzer mode (for separation of oxygen fromair along the cathode fluid flow path) is schematically depicted in FIG.2. As shown in FIG. 2, in this mode of operation the electrochemicalcell receives principal inputs of air 24′, electricity 38, and (in thecase of a proton transfer electrolyte) water 40, and produces principaloutputs of oxygen 35, heat 42, NEA 27, and in the case of a protontransfer electrolyte, water as part of stream of NEA 27 and as part of aflow path for water and thermal management 44 (e.g., removing condensedwater from the NEA as the NEA cools such as in a condenser, or waterremoval with a membrane separator). A second operational mode, which canalso be referred to as a fuel cell mode, is schematically depicted inFIG. 3. As shown in FIG. 3, in this mode of operation theelectrochemical cell receives principal inputs of air 24′ and fuel 32′,and produces principal outputs of electricity 38, heat 42, NEA 27, andwater 44. In the case of a proton transfer electrolyte, the water output44 may be part of stream of NEA 27 and as part of a flow path for waterand thermal management 44, and in the case of a solid oxide electrolyte,the water output 44 as part of the anode exhaust 34 (FIG. 1).

In some embodiments, the electrochemical cell 10 can operate utilizingthe transfer of protons across the electrolyte 12. Exemplary materialsfrom which the electrochemical proton transfer electrolytes can befabricated include proton-conducting ionomers and ion-exchange resins.Ion-exchange resins useful as proton conducting materials includehydrocarbon- and fluorocarbon-type resins. Fluorocarbon-type resinstypically exhibit excellent resistance to oxidation by halogen, strongacids, and bases. One family of fluorocarbon-type resins having sulfonicacid group functionality is NAFION™ resins (commercially available fromE. I. du Pont de Nemours and Company, Wilmington, Del.). Alternatively,instead of an ion-exchange membrane, the electrolyte 12 can be comprisedof a liquid electrolyte, such as sulfuric or phosphoric acid, which maypreferentially be absorbed in a porous-solid matrix material such as alayer of silicon carbide or a polymer than can absorb the liquidelectrolyte, such as poly(benzoxazole). These types of alternative“membrane electrolytes” are well known and have been used in otherelectrochemical cells, such as phosphoric-acid fuel cells.

During operation of a proton transfer electrochemical cell in theelectrolyzer or first mode of operation, water at the anode undergoes anelectrolysis reaction according to the formula

H₂O→½O₂+2H⁺+2e ⁻  (1)

The electrons produced by this reaction are drawn from an electricalcircuit 18 powered by electric power source 20 connecting the positivelycharged anode 16 with the cathode 14. The hydrogen ions (i.e., protons)produced by this reaction migrate across the electrolyte 12, where theyreact at the cathode 14 with oxygen in the cathode flow path 24 toproduce water according to the formula

½O₂+2H⁺+2e ⁻→H₂O  (2)

Removal of oxygen from cathode flow path 24 produces nitrogen-enrichedair exiting the region of the cathode 14. The oxygen evolved at theanode 16 by the reaction of formula (1) is discharged as oxygen or anoxygen-enriched air stream as anode exhaust 34.

During operation of a proton transfer electrochemical cell in the fuelcell or second mode of operation, fuel (e.g., hydrogen) at the anodeundergoes an electrochemical oxidation according to the formula

H₂→2H⁺+2e ⁻  (3)

The electrons produced by this reaction flow through electrical circuit18 to provide electric power to electric power sink 22. The hydrogenions (i.e., protons) produced by this reaction migrate across theelectrolyte 12, where they react at the cathode 14 with oxygen in thecathode flow path 24 to produce water according to the formula (2).

½O₂+2H⁺+2e ⁻→H₂O  (2)

Removal of oxygen from cathode flow path 24 produces nitrogen-enrichedair exiting the region of the cathode 14. Any unreacted hydrogen thatexits anode 16 via anode exhaust flow path 34 can be recycled to fuelflow path 32 using an ejector or blower (not shown).

As mentioned above, the electrolysis reaction occurring at thepositively charged anode 16 requires water, and the ionic polymers usedfor a PEM electrolyte perform more effectively in the presence of water.Accordingly, in some embodiments, a PEM membrane electrolyte issaturated with water or water vapor. Although the reactions (1) and (2)are stoichiom0etrically balanced with respect to water so that there isno net consumption of water, in practice moisture will be removed by NEA24 (either entrained or evaporated into the nitrogen-enriched air) as itexits from the region of cathode 14. Accordingly, in some exemplaryembodiments, water is circulated past the anode 16 along an anode fluidflow path (and optionally also past the cathode 14). Such watercirculation can also provide cooling for the electrochemical cells. Insome exemplary embodiments, water can be provided at the anode fromhumidity in air along an anode fluid flow path in fluid communicationwith the anode. In other embodiments, the water produced at cathode 14can be captured and recycled to anode 16 (not shown). It should also benoted that, although the embodiments are contemplated where a singleelectrochemical cell is employed, in practice multiple electrochemicalcells will be electrically connected in series with fluid flow to themultiple cathode and anode flow paths routed through manifoldassemblies.

In some embodiments, the electrochemical cell 10 can operate utilizingthe transfer of oxygen anions across the electrolyte 12. Exemplarymaterials from which the electrochemical oxygen anion-transportingelectrolytes can be fabricated include solid oxides such asyttria-stabilized zirconia and/or ceria doped with rare earth metals.These types of materials are well known and have been used in solidoxide fuel cells (SOFC).

During operation of an oxygen anion transfer electrochemical cell in theelectrolyzer or first mode of operation, oxygen at the cathode undergoesan electrochemical reduction reaction according to the formula

½O₂+2e ⁻→O⁼  (4)

The electrons consumed by this reaction are drawn from an electricalcircuit 18 powered by electric power source 20 connecting the positivelycharged anode 16 with the cathode 14. The oxygen anions produced by thisreaction migrate across the electrolyte 12, where they undergo anelectrochemical oxidation reaction at the anode 14 according to theformula

O⁼→½O₂+2e ⁻  (5)

Removal of oxygen from cathode flow path 24 produces nitrogen-enrichedair exiting the region of the cathode 14. The oxygen produced at theanode 16 by the reaction of formula (5) is discharged as oxygen or anoxygen-enriched air stream as anode exhaust 34.

During operation of an oxygen ion transfer electrochemical cell in thefuel cell or second mode of operation, oxygen at the cathode undergoesan electrochemical reduction reaction according to the formula

½O₂+2e→O⁼  (4)

The electrons consumed by this reaction are drawn from electronsliberated at the anode, which flow through electrical circuit 18 toprovide electric power to electric power sink 22. The oxygen anionsproduced by this reaction migrate across the electrolyte 12, where theyreact with fuel such as hydrogen at the anode according to the formula

H₂+O⁼→H₂O+2e ⁻  (6)

Carbon monoxide (e.g., contained in fuel reformate) can also serve asfuel in solid oxide electrochemical cells. In this case, the oxygenanions produced at the cathode according to formula (4) migrate acrossthe electrolyte 12 where they react with carbon monoxide at the anodeaccording to the formula

CO+O⁼→CO₂+2e ⁻  (7)

Removal of oxygen from cathode flow path 24 produces nitrogen-enrichedair exiting the region of the cathode 14. The steam and carbon dioxideproduced at the anode 16 by the reactions of formulas (6) and (7)respectively is discharged along with unreacted fuel as anode exhaust34. The unreacted fuel that exits anode 16 via anode exhaust flow path34 can be recycled to fuel flow path 32 using an ejector or blower (notshown). It can also be fed to a fuel processing unit wherein the steamcontributes to reforming.

Depending on whether a proton transfer cell or an oxygen ion transfercell is utilized, several technical issues can arise. In the case of aproton transfer cell, the NEA produced at the cathode can containsignificant amounts of water. Accordingly, in some embodiments, a systemutilizing a proton transfer cell can include a dryer in fluidcommunication with the cathode exhaust 26 bound for the ullage. Examplesof dryers can include a desiccant, a heater, a heat absorption side of aheat exchanger, a separation membrane, or combinations thereof. In someembodiments, the water removed from the cathode exhaust 26 is recoveredfor use at the anode during the electrolyzer or first mode of operation.

In the case of an oxygen ion transfer cell, a technical issue of heatmanagement can arise, as solid oxide electrochemical cells typicallyoperate at high temperatures (e.g., up to 1000° C.). One of the mostcommon techniques for heat removal from a solid oxide fuel cell is byincreasing air flow across the cathode; however, an increase in thecathode air flow rate will also tend to increase the oxygenconcentration of the NEA exiting the region of the cathode, which maylimit its potential impact for removing heat. During operation in theelectrolyzer or first mode, heat can be absorbed by increasing air flowacross the anode side of the solid oxide electrolyte 12′. Other coolingmodalities can be utilized for operation in the fuel cell or secondmode. In some embodiments, a system utilizing an oxygen ion transferelectrochemical cell can include a cooling gas flow path that isisolated from the cathode flow path. Example embodiments of thisapproach are schematically depicted in FIGS. 4 and 5, where a stack offuel cells formed by repeating modules comprising solid oxideelectrolyte 12′ and bipolar separators (e.g., separator plates) 50/50′.The alignment of the electrodes (not shown) on each solid oxideelectrolyte 12′ is indicated by the flow direction of O⁼ anions, withthe cathode on the right of hand side of each solid oxide electrolyte12′ and the anode on the left hand side of each solid oxide electrolyte12′, as depicted in FIGS. 4 and 5. Electrical circuit 18 is connected toelectric power sink 22 for operation in the fuel cell or second mode ofoperation.

In the example embodiment of FIG. 4, a flow of cooling air 48, includingany air flow having a lower temperature than the core temperature of theelectrochemical cell stack, is directed into one or more internalpassages (not shown) in the bipolar separators 50, where it absorbs heatfrom the higher surrounding temperatures. The cooling air flow 52 can beexhausted to the outside or can be used onboard for furtherthermodynamic processing. In the example embodiment of FIG. 5, one ormore internal passages (not shown) in the bipolar separators 50′ caninclude a reforming catalyst (e.g., nickel, noble metals, etc.) toprovide heat absorption by an endothermic steam reforming process. Inoperation, the reforming process is provided with a hydrocarbon fuel 54,which is mixed with steam contained in the portion of anode exhaust gaspumped by blower 64 from anode exhaust manifold 62, and directed intomanifold 56, from which it is directed into internal passages in thebipolar separators 50′ where it contacts the reformer catalyst and isheated by surrounding temperatures, resulting in an endothermic steamreforming reaction that absorbs heat from the stack core and removes itwith the flow of reformate exiting the internal passages of the bipolarseparators 50′. The reformate fluid exiting the bipolar separators 50′enters reformate manifold 58, from where it is directed to manifold 60as fuel for the anodes. In another embodiment, the reformate leaves thebipolar separators and directly enters the anodes. This is advantageouswhen each cell has a reforming bipolar separator, however, for reasonsof energy balance and cost, one reforming bipolar separator may servenumerous cells thus requiring the aforementioned reformate manifold 58.At the anodes, hydrogen in the reformate reacts with oxygen anions toform water, and carbon monoxide in the reformate reacts with oxygenanions to form CO₂ plus water. Both oxidation reactions involve atransfer of electrons which are collected from the anode and passedthrough electric circuit 18.

Another technical issue with solid oxide electrochemical cells andproton transfer electrochemical cells relates to providing air flow tothe cathode at target pressures. An example embodiment for providingcompressed air is schematically depicted in FIG. 6. As shown in FIG. 6,a turbocompressor comprising a compressor section 64, a turbine section66, and a motor assist section 68 is integrated with an electrochemicalcell having features identified as in FIG. 1. For ease of illustration,a single chemical cell is depicted in FIG. 6 integrated with aturbocompressor, but it is understood that a stack comprising multiplecells could be used instead of a single cell. Air flowing to the cathodealong cathode flow path 24 is compressed in compressor section 64, andthe NEA exiting from cathode at cathode exhaust 26 is expanded inturbine section 66. Work extracted by turbine 66 is used along with anassist from motor section 68 to drive the compressor section 64. Anodeside fluid flows are as depicted in FIG. 1, except that in the case of asolid oxide electrochemical cell, some of the anode exhaust along anodeexhaust flow path 34 can be directed along flow path 70 to the cathodeflow path 24 in fuel cell mode to pre-heat the air on the cathode flowpath 24 to promote migration of oxygen ions across the electrolyte 12and to promote higher power output from turbine section 68.

In some embodiments, the system can be operated in the first orelectrolyzer mode during conditions when the aircraft does not requirethe production of electrical power by the electrochemical cell (e.g.,normal operating conditions), and in the second or fuel cell mode duringa designated aircraft operating condition requiring the production ofelectrical power by the electrochemical cell (e.g., Emergency PowerSystem (EPS), Fire Suppression Low Rate of Discharge (LRD). In someembodiments, in addition to supplying NEA to the ullage of the fueltank(s) onboard the aircraft and electricity to onboard systems andcomponents when needed, the NEA may be also be used for other functions,such as serving as a fire-suppression agent. For example, cargocompartments, engines, and toilet waste bins on board aircraft typicallyhave fire-suppression systems that include a dedicated gas-distributionsystem comprising tubes routed to nozzles in the cargo bay to deployfire-suppression agents in the event of a fire. A variety offire-suppression agents may be deployed depending on the type and extentof the fire. In the case of a fire, all or some of the NEA could berouted to one or more of these fire-suppression distribution systems.This may be especially beneficial during the aircraft descent when thecargo bay is becoming re-pressurized to reduce the ingress of oxygenthat can feed a fire. In this case, the system may be operated toproduce NEA at the maximum flow rate. The NEA could also be used toenable inerting coverage over extended periods, which may be in additionto, or in lieu of, dedicated low-rate discharge inerting systems in thecargo bay(s). In some embodiments, this would have the technical effectof reducing the amount of ozone layer-depleting halogenated compounds,which are known greenhouse gases, required on the aircraft for firesuppression.

During operation, the system can be controlled to set fluid flow rates(e.g. air, fuel, or water feed rates) and the current or voltage levelsrequired by the electric power sink 22 in the first mode of operation orproduced by electric power source 20 in the second mode of operation, toproduce varying amounts of NEA in response to system parameters. Suchsystem parameters can include, but are not limited to temperature of thefuel in the aircraft fuel tank(s), oxygen content of the fuel in thefuel tanks, oxygen content of vapor in the ullage of fuel tanks,temperature rise in an enclosed space such as a cargo hold or avionicsbay, smoke and/or flame detection in said enclosed spaces, andtemperature and/or pressure of vapor in the ullage of fuel tanks, andother on-board parameters such as temperature, oxygen content, and/orhumidity level of air feed to the electrochemical cell. Accordingly, insome embodiments, the inert gas management system such as shown in FIGS.3 and 4 can include sensors for measuring any of the above-mentionedfluid flow rates, temperatures, oxygen levels, humidity levels, orcurrent or voltage levels, as well as controllable output fans orblowers, or controllable fluid flow control valves or gates. Thesesensors and controllable devices can be operatively connected to thecontroller 36, which can be an independent controller dedicated tocontrolling the inert gas management system or the electrochemical cell,or can interact with other onboard system controllers or with a mastercontroller. In some embodiments, data provided by the controller of theinert gas management system can come directly from a master controller.

While the present disclosure has been described in detail in connectionwith only a limited number of embodiments, it should be readilyunderstood that the present disclosure is not limited to such disclosedembodiments. Rather, the present disclosure can be modified toincorporate any number of variations, alterations, substitutions orequivalent arrangements not heretofore described, but which arecommensurate with the spirit and scope of the present disclosure.Additionally, while various embodiments of the present disclosure havebeen described, it is to be understood that aspects of the presentdisclosure may include only some of the described embodiments.Accordingly, the present disclosure is not to be seen as limited by theforegoing description, but is only limited by the scope of the appendedclaims.

1. An on-board aircraft electrochemical system, comprising: anelectrochemical cell comprising a cathode and an anode separated by anelectrolyte separator; a cathode fluid flow path in fluid communicationwith the cathode that discharges nitrogen-enriched air; anitrogen-enriched air flow path that receives nitrogen-enriched air fromthe cathode fluid flow path and delivers it to a fuel tank, a firesuppression system, or both a fuel tank and a fire suppression system; afirst anode fluid flow path configured to controllably direct water orair to the anode; a second anode fluid flow path configured tocontrollably direct fuel to the anode; an electrical connection incontrollable communication between an electric power source and thecathode and anode; an electrical connection in controllablecommunication between an electric power sink and the cathode and anode;and a controller configured to alternatively operate the electrochemicalcell in either: a first mode in which water or air is directed to theanode from the first anode fluid flow path, electric power is directedfrom the power source to the anode and cathode to provide a voltagedifference between the anode and the cathode, and nitrogen-enriched airis directed from the cathode to the fuel tank, the fire suppressionsystem, or both the fuel tank and the fire suppression system, or asecond mode in which fuel is directed to the anode from the second anodefluid flow path, electric power is directed from the anode and cathodeto the power sink, and nitrogen-enriched air is directed from thecathode to the fuel tank, the fire suppression system, or both the fueltank and the fire suppression system.
 2. The system of claim 1, whereinthe controller is configured to operate the electrochemical cellcontinuously in either the first mode or the second mode during aircraftoperation.
 3. The system of claim 1, wherein the nitrogen-enriched airflow path is configured to deliver nitrogen-enriched air to the fueltank.
 4. The system of claim 1, wherein the nitrogen-enriched air flowpath is configured to controllably deliver nitrogen-enriched air toeither or both of the fuel tank and the fire suppression system.
 5. Thesystem of claim 1, wherein the controller is configured to operate theelectrochemical cell in the first mode under aircraft operatingconditions that do not require the production of electrical power by theelectrochemical cell, and in the second mode during a designatedaircraft operating condition requiring the production of electricalpower by the electrochemical cell.
 6. The system of claim 1, comprisinga plurality of said electrochemical cells in electrical series in astack.
 7. The system of claim 1, wherein the electrolyte is selectedfrom a polymer electrolyte proton-transfer medium, a solid oxide, orphosphoric acid.
 8. The system of claim 1, wherein the electrolytecomprises a polymer electrolyte proton-transfer medium or phosphoricacid, and the first anode flow path is configured to controllably directwater to the anode.
 9. The system of claim 8, further comprising a gasdryer disposed along the ullage flow path to dry the nitrogen-enrichedair.
 10. The system of claim 8, wherein the electrolyte comprises apolymer electrolyte proton-transfer medium.
 11. The system of claim 1,wherein the electrolyte comprises a solid oxide, and the first anodeflow path is configured to controllably direct air to the anode.
 12. Thesystem of claim 11, further comprising a turbocompressor, wherein thecathode fluid flow path receives air from a compressor outlet of theturbocompressor and discharges air to a turbine inlet of theturbocompressor.
 13. The system of claim 11, comprising a cooling gasflow path isolated from the cathode flow path.
 14. The system of claim13, wherein the cooling gas flow path includes an air flow path on theanode side of the electrochemical cell during the first mode ofoperation.
 15. The system of claim 13, comprising a plurality of saidelectrochemical cells in a stack separated by electrically-conductivegas flow separators, wherein the cooling gas flow path includes an airflow path through a passage in one or more of the separators.
 16. Thesystem of claim 13, comprising a plurality of said electrochemical cellsin a stack separated by electrically-conductive gas flow separators,wherein the cooling gas flow path includes a fuel and steam flow paththrough a passage in one or more of the separators comprising areforming catalyst.
 17. The system of claim 12, comprising a pluralityof said electrochemical cells in a stack separated byelectrically-conductive gas flow separators, wherein the cooling gasflow path comprises an air flow path across an anode of one or moreelectrochemical cells in the stack configured or controlled to operatein the first mode of operation when the stack as a whole is operated inthe second mode of operation.
 18. A method of operating an on-boardaircraft electrochemical system, comprising selectively operating anelectrochemical cell comprising a cathode and an anode separated by anelectrolyte separator in either: a first mode in which water or air isdirected to the anode, electric power is provided to the anode andcathode to provide a voltage difference between the anode and thecathode, and nitrogen-enriched air is directed from the cathode to anaircraft fuel tank or an aircraft fire suppression system, or a secondmode in which fuel is directed to the anode, electric power is directedfrom the anode and cathode to one or more aircraft electricpower-consuming systems or components, and nitrogen-enriched air isdirected from the cathode to the aircraft fuel tank or aircraft firesuppression system.
 19. The method of claim 18, wherein theelectrochemical cell is operated continuously in either the first modeor the second mode during aircraft operation.
 20. The method of claim18, wherein the electrochemical cell is operated in the first mode undernormal aircraft operating conditions, and in the second mode during adesignated aircraft operating condition requiring the production ofelectrical power by the electrochemical cell.